121 Hermione
121 Hermione is a very large asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

 of the dark C spectral type
C-type asteroid
C-type asteroids are carbonaceous asteroids. They are the most common variety, forming around 75% of known asteroids, and an even higher percentage in the outer part of the asteroid belt beyond 2.7 AU, which is dominated by this asteroid type...

, orbiting in the Cybele group
Cybele asteroid
Cybele asteroids are a group of asteroids in the outer main belt with a semi-major axis between 3.27 AU and 3.7 AU, an eccentricity less than 0.3, and an inclination less than 25°. The group is named for the asteroid 65 Cybele...

 in the far outer asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

. As a C-type, it is probably composed of carbonaceous
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

 materials. It was discovered in 1872, and it also has small asteroid moon discovered in 2002.


Hermione was discovered by J. C. Watson
James Craig Watson
James Craig Watson was a Canadian-American astronomer born in the village of Fingal, Ontario Canada. His family relocated to Ann Arbor, Michigan in 1850....

 on May 12, 1872 from Ann Arbor, and named after Hermione
Hermione (mythology)
In Greek mythology, Hermione was the only daughter of Menelaus and Helen; however, some sources state that her mother also had a daughter, Iphigenia, by Theseus, as well as three younger sons, one by Menelaus, the others by Paris, prince of Troy.Hermione was nine when her mother left with Paris...

, daughter of Menelaus
Menelaus may refer to;*Menelaus, one of the two most known Atrides, a king of Sparta and son of Atreus and Aerope*Menelaus on the Moon, named after Menelaus of Alexandria.*Menelaus , brother of Ptolemy I Soter...

 and Helen in Greek mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

. Hermione is a Cybele asteroid
Cybele asteroid
Cybele asteroids are a group of asteroids in the outer main belt with a semi-major axis between 3.27 AU and 3.7 AU, an eccentricity less than 0.3, and an inclination less than 25°. The group is named for the asteroid 65 Cybele...

 and orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

s beyond most of the main-belt asteroids.

Physical properties

The asteroid has a bi-lobed shape, as evidenced by adaptive optics
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...

 images, the first of which were taken in December 2003, with the Keck telescope. Of several proposed shape models that agreed with the images, a "snowman"-like shape was found to best fit the observed precession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...

 rate of Hermione's satellite. In this "snowman" model, the asteroid's shape can be approximated by two partially overlapping spheres of radii 80 and 60 km, whose centers are separated by 115 km. A simple ellipsoid shape was ruled out.

Observation of the satellite's orbit has made possible an accurate determination of Hermione's mass. For the best-fit "snowman" model, the density is found to be 1.8 ± 0.2 g/cm³, giving a porosity of the order of 20%, and possibly indicating that the main components are fractured solid bodies, but that the asteroid is rather not a rubble pile
Rubble pile
In astronomy, rubble pile is the informal name for an object that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity...


An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...

s by Hermione have been successfully observed three times so far, the last time in February 2004.

Satellite (S/2002 (121) 1)

A satellite
Asteroid moon
A minor planet moon is an astronomical body that orbits a minor planet as its natural satellite. It is thought that many asteroids and Kuiper belt objects may possess moons, in some cases quite substantial in size...

 of Hermione was discovered in 2002 with the Keck II telescope, and is about 8 miles (13 km) in diameter. The satellite is currently designated S/2002 (121) 1. It has not yet been officially named, but "LaFayette" has been proposed by a group of astronomers in reference to the frigate used in secret by the Marquis de Lafayette to reach America to help the insurgents.

External links

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